1. Field of the Invention
The present invention relates to new and improved processes for inhibiting corrosion of ferrous metal surfaces (susceptible to corrosion) in the presence of an aqueous medium. More particularly, this invention relates to new and improved processes for the use of corrosion inhibiting amino acids effective to inhibit corrosion of ferrous metals under use conditions in the presence of an otherwise corrosive aqueous medium.
2. Description of the Related Art
An important mechanism for protecting the metal against corrosive deterioration is achieved through the use of inhibitors. Unfortunately, certain common corrosion inhibitors such as nitrogen- and aromatic compound-containing formulations, used widely as additives for inhibiting corrosion in aqueous heating and cooling systems, have been found to be hazardous to public health and to the surrounding environment. Removal of such hazardous compounds by precipitation or other treatments is complicated and expensive. Other corrosion inhibitors, such as chromatic salts have been banned from use because they are suspected carcinogens. Consequently, it has become desirable to examine the inhibition properties of biologically compatible and/or biodegradable compounds. Such compounds, if nontoxic, easy to produce in high purities, and biodegradable, can dramatically ease the chore of removal or recycling. Amino acids have been proposed for limited use.
For example, Nippon Kokoh, in Japanese Patent No. J50091546-A, July 22, 1975, disclosed that mixtures requiring both amines and amino acids or their salts, when dissolved in water to form 20% aqueous solutions, inhibited atmospheric corrosion of various ferrous and non-ferrous metal sheets. The pH of the moisture absorbed on the sheets is believed to have been approximately 5.5 or less.
However, more extensive studies on common amino acids alone have not proven promising. For example, in V. Hluchan et al, "Amino Acids As Corrosion Inhibitors in Hydrochloric Acid Solutions," Warkstoffe und Korrosion, 39, 512-517 (1988) 22 of the most common amino acids were investigated as inhibitors for the corrosion of iron in 1.0 M hydrochloric acid, at pH of about 0. Generally, those having inhibiting characteristics at acid pH did not demonstrate corrosion inhibition efficiencies effective for immediate industrial use. The longer hydrocarbon chain amino acids and those having additional amino groups, or groups which could increase electron density on the amino groups, demonstrated the only tendency toward effective corrosion inhibition.
Notably, aspartic acid, the preferred amino acid for use in the present invention, and glutamic acid did not come within the scope of the "tendency". The conclusion was that such amino acids are particularly poor inhibitors because of the single amino group, the short carbon chain and the additional carboxyl group.
Moreover, it is considered a drawback by those skilled in the art to employ aspartic acid as an inhibitor at above acid pH conditions because aspartic acid is known to be inherently corrosive at slightly alkaline pH conditions. See K. Ramakrishnaiah, "Role of Some Biologically Important Compounds on the Corrosion of Mild Steel and Copper in Sodium Chloride Solutions", Bulletin of Electrochemistry, 2(1), 7-10 (1986). Therein it was disclosed that aspartic acid at a pH of 8 actually accelerated corrosion (inhibition efficiency of -25.4%). In fact, even when combined with an excellent corrosion inhibitor for mild steel such as papaverine, the presence of aspartic acid maintained the solution's corrosiveness.
An associated problem in the industry is that fluid movement is known to increase the rate of corrosion for ferrous metals when exposed to an aqueous environment. Accordingly, whatever corrosive effect which might be anticipated from amino acids such as aspartic acid in aqueous media would be expected to worsen, as a practical matter, if such amino acids were present in automotive, cooling, or heating devices where such media would be set in motion.
Therefore, amino acids such as aspartic acid, although nontoxic and biodegradable, have been avoided as corrosion inhibitors.
A process for inhibition of corrosion of ferrous metals by using amino acids having only a single amino group, and having an additional carboxyl group (such as aspartic acid) under conditions wherein such amino acids are fully ionized would represent a surprisingly unexpected discovery while satisfying a long-felt need in the industry. Likewise, a corrosion inhibitor for ferrous metals which would decrease the rate of corrosion, even under increased aqueous fluid movement conditions, would represent a substantial improvement in the art.